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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.operator.alternative"></a><a class="link" href="alternative.html" title="Alternative Parser (a | b)">Alternative
          Parser (<code class="computeroutput"><span class="identifier">a</span> <span class="special">|</span>
          <span class="identifier">b</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h0"></a>
            <span class="phrase"><a name="spirit.qi.reference.operator.alternative.description"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.description">Description</a>
          </h6>
<p>
            The alternative operator, <code class="computeroutput"><span class="identifier">a</span>
            <span class="special">|</span> <span class="identifier">b</span></code>,
            matches one of two or more operands (<code class="computeroutput"><span class="identifier">a</span></code>,
            <code class="computeroutput"><span class="identifier">b</span></code>, ... etc.):
          </p>
<pre class="programlisting"><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="special">...</span>
</pre>
<p>
            Alternative operands are tried one by one on a first-match-wins basis
            starting from the leftmost operand. After a successfully matched alternative
            is found, the parser concludes its search, essentially short-circuiting
            the search for other potentially viable candidates. This short-circuiting
            implicitly gives the highest priority to the leftmost alternative.
          </p>
<p>
            Short-circuiting is done in the same manner as C or C++'s logical expressions;
            e.g. <code class="computeroutput"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span> <span class="special">&lt;</span> <span class="number">3</span> <span class="special">||</span> <span class="identifier">y</span>
            <span class="special">&lt;</span> <span class="number">2</span><span class="special">)</span></code> where, if <code class="computeroutput"><span class="identifier">x</span>
            <span class="special">&lt;</span> <span class="number">3</span></code>,
            the <code class="computeroutput"><span class="identifier">y</span> <span class="special">&lt;</span>
            <span class="number">2</span></code> test is not done at all. In addition
            to providing an implicit priority rule for alternatives which is necessary,
            given its non-deterministic nature, short-circuiting improves the execution
            time. If the order of your alternatives is logically irrelevant, strive
            to put the (expected) most common choice first for maximum efficiency.
          </p>
<h6>
<a name="spirit.qi.reference.operator.alternative.h1"></a>
            <span class="phrase"><a name="spirit.qi.reference.operator.alternative.header"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.header">Header</a>
          </h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/qi/operator/alternative.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_alternative</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
            Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>.
          </p>
<h6>
<a name="spirit.qi.reference.operator.alternative.h2"></a>
            <span class="phrase"><a name="spirit.qi.reference.operator.alternative.model_of"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.model_of">Model of</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a>
            </p></blockquote></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code>, <code class="computeroutput"><span class="identifier">b</span></code></span></dt>
<dd><p>
                  A <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>
                </p></dd>
</dl>
</div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h3"></a>
            <span class="phrase"><a name="spirit.qi.reference.operator.alternative.expression_semantics"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.expression_semantics">Expression
            Semantics</a>
          </h6>
<p>
            Semantics of an expression is defined only where it differs from, or
            is not defined in <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Semantics
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">a</span> <span class="special">|</span>
                      <span class="identifier">b</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      Match <code class="computeroutput"><span class="identifier">a</span></code> or
                      <code class="computeroutput"><span class="identifier">b</span></code>.
                    </p>
                  </td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h4"></a>
            <span class="phrase"><a name="spirit.qi.reference.operator.alternative.attributes"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.attributes">Attributes</a>
          </h6>
<p>
            See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.qi.quick_reference.compound_attribute_rules.notation">Compound
            Attribute Notation</a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Attribute
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">a</span> <span class="special">|</span>
                      <span class="identifier">b</span></code>
                    </p>
                  </td>
<td>
                    <p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">c</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="identifier">c</span><span class="special">):</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">Unused</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">A</span></pre>
<p>
                    </p>
                  </td>
</tr></tbody>
</table></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              Alternative parsers do not roll back changes made to the outer attribute
              because of a failed alternative. If you need to enforce that only the
              succeeded alternative changes the outer attribute please utilize the
              directive <a class="link" href="../directive/hold.html" title="Parser Directive for Attribute Commit/Rollback (hold[])"><code class="computeroutput"><span class="identifier">hold</span></code></a><code class="computeroutput"><span class="special">[]</span></code>.
            </p></td></tr>
</table></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h5"></a>
            <span class="phrase"><a name="spirit.qi.reference.operator.alternative.complexity"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.complexity">Complexity</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              The overall complexity of the alternative parser is defined by the
              sum of the complexities of its elements. The complexity of the alternative
              parser itself is O(N), where N is the number of alternatives.
            </p></blockquote></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h6"></a>
            <span class="phrase"><a name="spirit.qi.reference.operator.alternative.example"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.example">Example</a>
          </h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a>
              section.
            </p></td></tr>
</table></div>
<p>
            Some using declarations:
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">;</span>
</pre>
<p>
          </p>
<p>
            Simple usage:
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span>
<span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span>
</pre>
<p>
          </p>
<p>
            Extracting the attribute variant (using <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>):
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">attr</span><span class="special">;</span>
<span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span>
</pre>
<p>
          </p>
<p>
            This should print <code class="computeroutput"><span class="string">"Hello"</span></code>.
            Note: There are better ways to extract the value from the variant. See
            <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>
            visitation. This code is solely for demonstration.
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(&amp;</span><span class="identifier">attr</span><span class="special">))</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">else</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;(</span><span class="identifier">attr</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
          </p>
<p>
            Extracting the attributes using Semantic Actions with <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>
            (this should print <code class="computeroutput"><span class="number">123</span></code>):
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="special">(</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">)[</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">_1</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">]);</span>
</pre>
<p>
          </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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